The angular momentum (AM) of light has contributions from both the spin angular momentum (SAM) and the orbital angular momentum (OAM) components. The former is dependent on the polarization of the light, with a purely left- and right-handed polarized beam carrying SAM of \(\hbar \)  and \(-\hbar \)  per photon, respectively. The OAM is related to the spatial structure of the beam, specifically, a vortex phase of topological charge \(\ell \)  results in OAM of \(\ell \hbar \)  per photon [1]}, [2]}, [3]}. Over the past few years, there has been enormous interest in the interactions between SAM and OAM due to its fundamental importance [4]}, [5]}, [6]} and emerging applications in nanophotonics [7]}, [8]}, [9]}. Spin-orbit (SO) interaction often occurs in non-paraxial beams including evanescent waves [10]}, [11]}, scattering [12]}, [13]}, [14]}, and tight focusing processes [15]}, [16]}. Recently, SO interaction was reported in counter-propagating paraxial beams in free-space [17]}, but could not explain why the total electric AM was not conserved. In this paper, we propose that the non-conservation of electric angular momentum, including SAM, OAM and total AM, is a general phenomenon in counter-propagating beams. We attribute this phenomenon to spin-spin and orbit-orbit interaction between the electric and magnetic fields. Previously it has been shown that the electric and magnetic fields contribute equally in travelling paraxial beams [18]}, but now we make clear that while this is true, it is only so for travelling waves; counter-propagating beams do not obey this rule. We support our hypothesis by a theoretical framework that is validated by experiment, and offer a new perspective on spin-spin and orbit-orbit coupling in propagating light.
